Medical science has long sought effective treatments for disease conditions involving stenosis (narrowing or obstruction) of the lumen of an artery. This condition, known generally as an occlusion, occurs in patients suffering from atherosclerosis, which is characterized by an accumulation of fibrous, fatty or calcified tissue in the arteries, otherwise known as atheromata or plaques. An occlusion may be partial or total; it may be soft and pliable or hard and calcified. Occlusions can arise at a great variety of sites in the arterial system including the aorta, the coronary and carotid arteries, and peripheral arteries. An occlusion can result in hypertension, ischemia, angina, myocardial infarction, stroke and even death.
Minimally invasive procedures are the preferred treatment of arterial occlusions. In these procedures, a catheter—a long, highly flexible tubular device—is introduced into a major artery through a small arterial puncture made in the groin, upper arm, upper leg, or neck. The catheter is advanced and steered into the site of the stenosis. A great variety of devices have been developed for treating the stenosed artery, and these devices are placed at the distal end of the catheter and delivered thereby. Example procedures include percutaneous transluminal coronary angioplasty (PTCA), directional coronary atherectomy (DCA), and stenting.
In a total occlusion, a passageway must first be opened through the occlusion to allow the balloon/stent catheter to be placed in the target stenosed segment of the vessel. As occlusion morphology is complicated and varies from patient to patient, common methods and devices for opening these occlusions have had limited success and require long procedures with potentially adverse effects on the patient. Such adverse effects include perforation of blood vessel wall, high radiation dose or damage to kidneys due to extensive use of angiographic contrast material.
Stenoses, or occlusions, are made of a variety of materials—from softer fatty substances such as cholesterol, to tougher fibrous material, to hard calcified material. Generally the ends of the occlusion—the proximal and distal caps—comprise the harder calcified material. The harder materials are more difficult to penetrate, requiring a significant amount of energy, the softer materials require less energy. Therefore, opening an occlusion requires transfer of relatively extensive energy to the distal end of a catheter or guide wire, especially when calcification is present.
Some available methods for opening total occlusions are radio-frequency ablative energy (as used in the system sold by Intralumenal Therapeutics as Safecross™), vibrational energy of about 20 kHz and small amplitudes (as used in the system sold by FlowCardia Inc. as Crosser™), dedicated stiff guide wire which pushes a passage through the occlusion (as developed by Asahi Intec Co. and distributed as Confianza 9 g/Conquest and Miracle 12 g guide wires) and mechanical vibration elements working at high frequency (FlowCardia Inc.'s Crosser™). The latter means for opening occlusions suffer from significant energy loss between the energy source at the proximal end of the catheter and the driller located at the distal end of the catheter, as well as limited working life due to material fatigue. For example, with an ultrasound catheter, the ultrasonic energy usually originates from an ultrasound transducer at the proximal end of the catheter and is then transmitted to the distal head of the catheter as a sinusoidal wave, causing the distal head to vibrate and either ablate or disrupt the target occlusion. To reach treatment sites, such catheters must be rather long—about 90-150 cm or more—and therefore a large amount of energy must initially be transmitted to reach the distal end. At the same time, to be flexible enough to course through highly tortuous vessels, the catheter must be reasonably thin. The long length and narrow diameter combine to make wire breakage a common problem due to the stress and wear from the high energy pulses. Guide wires stiff enough to penetrate hard occlusions have the disadvantage that their inflexibility and straight tips make navigating through tortuous vessels difficult and increase the risk of vessel perforation. Rigid materials that are sufficiently flexible to accommodate the highly tortuous vessels have the problem of buckling, due to the proximal location of the pushing source. Buckling results in energy loss by transfer to transverse forces and friction against the lumen housing the rigid material. All such devices provide limited success rate ranging from 40-70%.
Occlusions comprise a variety of materials of different density and hardness. Therefore, the nature of the energy used in a re-canalization device should suit the specific occlusion and the penetration should be controlled to prevent perforation of the artery walls or damage to healthy tissue. Additionally, because the energy originates at the proximal end of the catheter it must be able to reach the distal end of the device near the occlusion at a level sufficient to effect penetration of the occlusion without damaging the conductive wires and without sacrificing flexibility of the device. As previously described, current devices suffer either from an insufficient amount of energy transferred to the distal end of the device or a mismatch between the type of energy delivered and the type of occlusion, sometimes resulting in too much force being applied and thereby increasing the risk of damage, or even perforation, of the lumen wall. Accordingly, there is a need for a system or apparatus that can transfer adequate energy to the re-canalization device.
In endolumenal devices designed for penetrating vessel occlusions, mechanical movement, i.e., oscillation, of the member that contacts the occlusion is usually generated by placing an energy source at the proximal end of the device and transferring the energy to the distal end of the device by mechanical means. For example, one prior art device (i.e., FlowCardia Inc.'s Crosser™) uses a rigid Nitinol wire. The rigidity of the wire permits an axial force initiated at the proximal end of the wire to be transmitted to the distal end of the wire, by pushing the wire. However, such energy transfer mechanisms suffer from significant, yet unpredictable (i.e., variable), energy loss due to energy transfer to the housing tube (e.g., catheter lumen). This is a particular problem when the rigid wire bends to conform to the anatomy of the blood vessel. Energy loss of rigid wires are due mainly to two mechanisms: (1) Moment of inertia, which may be illustrated by bending a rigid body. The force imposed to bend the rigid wire is transferred to friction when the rigid wire is housed within a catheter lumen. (2) Buckling of the wire, a situation that causes the axial force to be shifted to transverse forces and results in increased friction forces within the housing lumen. Further, if the axial force is increased to compensate for the energy losses, the buckling is exacerbated, making axial oscillation, and in particular controllable axial oscillation, even more difficult to achieve.
An important engineering phenomenon is the buckling of slender beams upon load. The critical force required to buckle a slender beam (including, for example, a rigid wire) is given by Equation 1:
                                          F            c                    =                                                    π                2                            ⁢              EI                                                      (                KL                )                            2                                      ,                            (        1        )            where Fc is maximum force the rigid wire can support without buckling, L is the length of the rigid wire, and K is a numeric constant which depends on the way the rigid wire is supported at its ends. For example, if both ends are pinned (i.e., free to rotate), then K=1. If one end is pinned and the other end is fixed, then K=0.7. If a straight wire that is held at its distal end is pushed at its proximal end by a force exceeding the critical buckling force Fc, the rigid wire will buckle laterally, and will not transmit the pushing force ahead.
A rigid wire winding within a catheter lumen—in particular a catheter that courses through a tortuous blood vessel—will be bent. Even without pulling or pushing such a rigid wire, there are forces exerted upon the rigid wire to keep it bent. Friction created by the bent wire against the lumenal surface of the catheter causes the rigid wire to be pinned at some point. If the friction at the pinned point is larger than the buckling threshold, a buckling will occur and adversely affect the pushability of the wire. The resistance that a rigid wire meets at a vessel occlusion works the same way as a pinned point due to friction at a bend. A rigid wire in a tube such as a catheter will move only if the pushing force is larger than the friction force or resistance acting upon the rigid wire. If the length of the straight portion of the rigid wire preceding the point of resistance is long enough, however, the rigid wire will buckle before the pushing force becomes large enough to overcome the friction. This explains why it is difficult to transmit a force to one end of a winding rigid wire by pushing from the opposite end, because the rigid wire is expected to buckle.
Therefore, there is a need in the art for an apparatus for penetrating vessel occlusions that is capable of delivering efficient energy in a controlled and safe manner to open vessel occlusions, and to improve the deliverability of catheters carrying such devices through blood vessels. There also is a need for a system that both transfers adequate energy and can adjust the amount of energy transmitted to the penetrating end of the device based on the hardness of the occlusion.